Facile Water-Based Strategy for Synthesizing MoO3-x Nanosheets: Efficient Visible Light Photocatalysts for Dye Degradation.

Nanostructured molybdenum oxides are promising materials for energy storage, catalysis, and electronic-based applications. Herein, we report the synthesis of MoO3-x nanosheets (x stands for oxygen vacancy) via an environmentally friendly liquid exfoliation approach. The process involves the reflux of the bulk α-MoO3 precursor in water at 80 °C for 7 days. Electron microscopy and atomic force microscopy show that the MoO3-x nanosheets are a few nanometer thick. MoO3-x nanosheets exhibit near infrared plasmonic property that can be enhanced by visible light irradiation for a short time (10 min). Photocatalytic activity of MoO3-x nanosheets for organic dye decolorization is examined using two different dyes (rhodamine B and methylene blue). Under visible light irradiation, MoO3-x nanosheets make a rapid decolorization for the dye molecules in less than 10 min. The simple synthesis procedure of MoO3-x nanosheets combined with their remarkable photochemical properties reflect the high potential for using the nanosheets in a variety of applications.

Introduction

Inorganic two-dimensional materials such as transition metal oxides, carbides, and chalcogenides are attractive materials because of their unique physical and chemical properties.[1−7] Transition metal oxide nanosheets are applied for catalysis,[8−10] gas sensing,[11,12] and energy storage applications.[13−15] They can be synthesized using different methods including liquid exfoliation,[16,17] hydrothermal synthesis,[18] chemical and physical vapor depositions,[13,19] salt-template,[20] and others.[21−23] Among these approaches, liquid exfoliation of bulk metal oxides is the most common technique to fabricate transition metal oxide nanosheets.[16,17] However, liquid exfoliation usually involves the use of organic solvents, which is environmentally unfriendly and leads to undesirable wastes. Thus, large efforts have been focused on the development of environmentally friendly approaches.[21,22,24]

Molybdenum trioxide (MoO3) is a superior semiconducting material[25,26] which can be used in several applications, such as batteries,[27] supercapacitors,[28] gas sensors,[29] field effect transistors,[30] and catalysis.[31−33] In addition, MoO3 is a polymorph material with at least four known phases [orthorhombic (α-MoO3),[34] monoclinic (β-MoO3),[35] hexagonal (h-MoO3),[36] and high pressure monoclinic (MoO3-II)].[37] Among them, α-MoO3 has been extensively studied because of its outstanding electrochemical and catalytic activities.[38,39] So far, a variety of α-MoO3 nanostructures were synthesized including nanoparticles,[40,41] nanobelts,[9,11] flower-like hierarchical structures,[36] nanoflakes,[30,42−45] and nanosheets.[10,28,46−48] MoO3 nanosheets can be prepared using hydrothermal synthesis,[47] salt-templated approach,[20] and milling or sonication of bulk α-MoO3 powder mixed with organic solvents (e.g., N-methyl-2-pyrrolidone, ethanol, and isopropanol).[30,33,43] However, these methods include several steps,[29] produce undesirable organic wastes, and require a high temperature.[20]

Organic dyes are widely used in industries, especially in textile manufacturing; however, they are considered as environmental threats.[49] Water treatment approaches, such as adsorption,[50] photocatalysis,[51] or others,[52,53] were reported for the treatment of dyes in waste water. Several materials, including zeolitic imidazolate frameworks (ZIF-8),[54] Degussa TiO2 (P25), and ZnO,[55] were reported as photocatalysts for dye degradation, decolorization, or encapsulation.[56] One interesting property of α-MoO3 is being a semiconducting layered material, with a tunable band gap depending on the intercalating ions between layers.[57] Therefore, it can be used as a photocatalyst for dye degradation.[32,58−62] MoO3– nanosheets are efficient photocatalysts for dye degradation/decolorization because of their high aspect ratio and the presence of oxygen vacancies.[32,58−62] Furthermore, they can generate hydrogen peroxide (H2O2) upon dispersion in water which causes degradation of organic dyes.[36]

In this paper, we report the synthesis of MoO3– nanosheets using a one-step water-based exfoliation technique. The method provides a cheap and environmentally friendly strategy for fabricating MoO3– nanosheets with a reasonable yield. The nanosheets show a near infrared (NIR) plasmonic property that was mediated using visible light irradiation. The nanosheets were applied as a photocatalyst for decolorization of organic dyes.

Results and Discussion

Synthesis, Structural Properties, and Chemical Composition of MoO3– Nanosheets

The bulk α-MoO3 possesses a tunable layered structure with an orthorhombic unit cell.[38] Water molecules have an affinity to intercalate into the MoO3 layers,[63] which results in an expansion of the interlayer distances and thus weakens the bonds holding the α-MoO3 layers together. Consequently, the reflux of bulk α-MoO3 in water at an elevated temperature can lead to its exfoliation into the nanosheets. The exfoliation process is schematically illustrated in Scheme . In a typical experiment, bulk α-MoO3 was dispersed in deionized water to form a white suspension, which was then refluxed at 80 °C for 5–7 days. At the end of the reflux, the suspension color changes to yellow because of the formation of MoO3– nanosheets. The exfoliated nanosheets can be separated from the remaining bulk material by centrifugation at a high speed (10 000 rpm). The bulk material was decanted as a white precipitate, whereas the MoO3– nanosheets remain dispersed in the mother liquor, which has a faint yellow color and form green strips of MoO3– nanosheets upon drying in air at 80 °C for 2–5 h (Figure S1c). The yield of MoO3– nanosheets varied from 17 to 60% depending on the reflux temperature and the concentration of the starting bulk material. The yield increases with the increase of reflux temperature but decreases with the concentration of the starting material.

Scheme 1

Schematic Presentation for the Exfoliation Process of Bulk α-MoO3 to MoO3– Nanosheets

Powder X-ray diffraction (PXRD) patterns of the exfoliated MoO3– nanosheets show a poor crystallinity (Figure a). However, the resolved XRD peaks suggest that the exfoliated nanosheets are composed of both orthorhombic α-MoO3 and hydrated MoO3 (h-MoO3). Interestingly, the exfoliation of α-MoO3 occurs readily, even under dark conditions (via covering the reaction vessel with an aluminum foil), which reveals that the visible light is not playing a significant role in the exfoliation process (Figure a). In the following sections, MoO3– nanosheets were synthesized via refluxing at 80 °C for 7 days under natural light.

Figure 1

Structural properties and chemical composition of MoO3– nanosheets: (a) XRD patterns of exfoliated MoO3– nanosheets using 0.45 (in dark conditions), 0.90, and 1.80 g of bulk α-MoO3 and standard patterns for h-MoO3 and α-MoO3. (b) Raman spectra of exfoliated MoO3– nanosheets, bulk α-MoO3, and bulk MoO2. (c,d) XPS spectra of Mo 3d and O 1s core levels of MoO3– nanosheets, respectively.

Raman spectrum of the MoO3– nanosheets shows broad and low intense peaks compared to the bulk α-MoO3 (Figure b). The sharp Raman peaks associated with α-MoO3 at 283, 290, 336, 666, 820, and 994 cm–1 became less intense,[8,30,43,64] and the extra peaks at Raman shifts of 244, 370, 480, 865, and 962 cm–1 refer to MoO3–,[30,42,64] suggesting the creation of oxygen vacancies in the exfoliated nanosheets. Although the ratio of peak intensities at 283 and 290 cm–1 (I283/I290) can be used to estimate the amount of oxygen vacancies,[64] unfortunately, the Raman spectrum of MoO3– nanosheets is not well-resolved and thus it is difficult to make such an estimation.

X-ray photoelectron spectroscopy (XPS) analysis was employed to investigate the oxidation states and the presence of oxygen vacancies in the MoO3– nanosheets. The Mo 3d spectrum in Figure c displays a couple of peaks at 233.0 and 236.1 eV, which refer to the binding energies of Mo6+ 3d5/2 and Mo6+ 3d3/2, respectively. The energy gap between the two doublets is approximately 3.1 eV, which is consistent with the previous report.[65] The O 1s spectrum shows a broad asymmetric peak, which can be deconvoluted into three peaks, as shown in Figure d. The main peak (red curve) at 530.9 eV can be assigned as O2–. This value is slightly lower than that reported for commercial α-MoO3 (531.3 eV),[8] suggesting a change in the coordination environment between O and Mo atoms in the MoO3– nanosheets. In previous reports, the shift of the O 1s peak to a lower binding energy was attributed to the transfer of electrons to the oxygen vacancies.[8,66] The peak at 531.7 eV (blue curve) can be assigned as the surface-adsorbed species (OH–, O–, or oxygen vacancies).[8,67,68] Finally, the peak at 532.8 eV (orange curve) can be assigned to adsorbed water.[8,67,68]

Chemical Bonding, Photochemical Properties, and Morphology of MoO3– Nanosheets

The chemical bonds in MoO3– nanosheets and bulk α-MoO3 were compared using Fourier transform infrared (FTIR). The bulk α-MoO3 displays peaks at the wavenumbers of 434, 532, 810, 840, and 976 cm–1, whereas the MoO3– nanosheets show peaks at wavenumbers of 532, 654, 615, 810, 864, 960, 1600, and 3400 cm–1 (Figure a). The complete assignment of all peaks is described in Note S1 (see Supporting Information). The vibrational peaks between 1000 and 900 cm–1 refer to Mo=O bond characteristic stretching vibrations.[36] The small shift between bulk α-MoO3 (976 cm–1) and MoO3– nanosheets (960 cm–1) shows that the local structure surrounding the Mo core is different because of the oxygen vacancies of the nanosheets. Therefore, the FTIR spectra prove that the chemical bonding and local structure environment of the MoO3– nanosheets are different from those of the bulk α-MoO3, which confirms the successful exfoliation of the bulk MoO3.

Figure 2

Chemical bonding and photochemical properties of MoO3– nanosheets: (a) FTIR spectra of the exfoliated MoO3– nanosheets and bulk α-MoO3. (b) UV–vis absorbance and (c) NIR absorbance of the MoO3– nanosheet aqueous solution under visible light irradiation for a given interval. (d) Variation of the NIR plasmonic peak absorbance at the wavelengths of 1160 and 954 nm over the time.

The UV–vis spectrum for MoO3– nanosheets shows absorbance in the range of 320–800 nm, with a broad absorption peak at a wavelength of 320 nm (see Figure S1a). The absorbance in the visible range (400–800 nm) is attributed to the existence of d electrons arising from the intervalence charge transfer between Mo5+ and Mo6+,[69] which explains the turquoise (bluish-green) color of the MoO3– aqueous suspension compared to the white color of the bulk α-MoO3 suspension (see Figure S1b). The electron charge transfer can be further activated by solar light irradiation.[43−45] To investigate the effect of light irradiation on MoO3– nanosheets, we recorded the UV–vis–NIR absorbance of MoO3– under visible light irradiation (Figure b–d). The absorbance in the range of 500–800 nm shows a decrease in the peak intensity over the time (0–60 min). Furthermore, a couple of new weak absorption peaks emerged at the wavelengths of 954 and 1160 nm (Figure c). These two peaks refer to the localized surface plasmon resonance (LSPR) of MoO3– nanosheets.[30,43−45] The intensity of the two peaks increases with time and reaches its maximum after ∼10 min. The increase in the intensity of the NIR plasmonic peaks is due to the excitation caused by light irradiation which increases the free carrier density.[69] Thus, visible light irradiation causes a dramatic effect on the photochemical properties of MoO3– nanosheets. It is worth noting that the UV–vis–NIR spectra backgrounds decrease with time (see Figure b,c). If the spectra backgrounds were removed, there will be no dramatic change in the plasmonic peak intensities after 10 min of light irradiation.

Field emission scanning electron microscopy (FE-SEM) revealed that the exfoliated MoO3– and bulk α-MoO3 possess different morphologies. The bulk α-MoO3 has a plate-like morphology (Figure a), whereas the exfoliated MoO3– is composed of fine nanosheets with a few nanometer thickness (Figure b,c). Transmission electron microscopy (TEM) and atomic force microscopy (AFM) further confirmed the uniform nanosheet morphology (Figure d,e). The AFM height profile for the MoO3– nanosheets showed that the nanosheet thickness is about 10 nm (Figure f). Furthermore, the aqueous suspension of MoO3– showed Tyndall effect (inset Figure c), which reflects the colloidal nature of the suspension and indicates the formation of homogeneous and stable MoO3– nanosheets.

Figure 3

Morphology of MoO3– nanosheets: (a) FE-SEM image of the bulk α-MoO3. (b,c) FE-SEM images of MoO3– nanosheets at different magnifications. Inset in (c) shows a photograph image for the Tyndall effect of the aqueous suspension of MoO3– nanosheets. (d,e) TEM and AFM images of the MoO3– nanosheets. (f) Height profile of the highlighted dashed lines in (e).

Photocatalytic Activity of Bulk α-MoO3 Versus MoO3– Nanosheets for Organic Dye Decolorization

The photocatalytic decolorization/degradation process of Rhodamine B (RhB) and methylene blue (MB) using bulk α-MoO3 or MoO3– nanosheets was carried out under visible light irradiation with stirring. The decolorization process was tracked via recording the UV–vis absorption at a given reaction time. The UV–vis absorption spectra of the pure dye solution under visible light irradiation displayed insignificant changes over the time (Figures a and 5a). The absorption maxima at 550 and 515 nm for RhB (Figure a) or at 665 and 605 nm for MB (Figure a) refer to the monomeric (0–0 band) and dimeric (0–1 band) forms of the corresponding dye. Notably, the intensities of the absorption peaks of the investigated dye (RhB or MB) decreased in the presence of bulk α-MoO3 (Figures b and 5b) or MoO3– nanosheets (Figures c and 5c), which indicates the photodegradation of the corresponding dye. Furthermore, in the presence of MoO3– nanosheets, the shape of the absorption peaks changed after the direct addition of the nanosheets to the investigated dye (see the red curves in Figures c and 5c). This observation indicates that the reaction is fast. As reported in previous investigations using h-MoO3[36] or α-MnO2/h-MoO3 hybrid material,[32] the dye degradation can be attributed to the strong chemisorption between the MoO3– nanosheets and the dye, in addition to the formation of reactive radical species such as OH• and O2•–. The peak broadening upon a photocatalytic reaction with MoO3– nanosheets indicates the change of the chromophoric group of the dye under investigation. To the best of our knowledge, this observation is reported for the first time. Taking into account of the low surface areas of bulk MoO3 and MoO3– (Brunauer–Emmett–Teller and Langmuir surface areas are 4 and 6 m2/g for bulk MoO3 and 2 and 3 m2/g for MoO3–, respectively, Figure S2), the reactive radical species play a leading role for dye degradation. The color changes of RhB dye were also monitored using naked eye, as shown in Figure S3.

Figure 4

UV–vis absorbance plots for RhB photocatalytic degradation: (a) RhB without the catalyst, (b) RhB with bulk α-MoO3, (c) RhB with MoO3– nanosheets, and (d) variation of the removal efficiency of RhB in the presence of the catalyst (α-MoO3 or MoO3– nanosheets) over the time.

Figure 5

UV–vis absorbance plots for MB photocatalytic degradation: (a) MB without catalyst (b) MB with bulk α-MoO3, (c) MB with MoO3– nanosheets, and (d) variation of the removal efficiency of MB in the presence of the catalyst (α-MoO3 or MoO3– nanosheets) over the time.

Generally speaking, there are many pathways for dye photodegradation, such as “step-by-step N-dealkylation” and “cycloreversion” (cleavage of the whole chromophore structure).[70,71] The former pathway usually shows a shift in the absorption maximum, whereas the latter pathway does not make a shift in the absorption maximum but it causes a decrease in the absorption intensity.[70,71] On the basis of the recorded UV–vis absorption spectra (Figures and 5), both pathways were observed for MoO3– nanosheets, whereas the bulk α-MoO3 showed only a cycloreversion pathway. It is worth mentioning that we observed 10% degradation for RhB under dark conditions (no light) using MoO3– nanosheets as the catalyst (Figure S4). This observation agrees well with previous report, which showed the degradation of the organic dye in the absence of light using h-MoO3.[36] This phenomenon was attributed to the in situ generation of H2O2, which is the major oxidizing agent for dye degradation under dark conditions.[36]

The degradation products of the investigated dyes were probed using FTIR (Figure ) and electrospray ionization mass spectrometry (ESI-MS) (Figure S5). The FTIR spectra after photocatalytic reactions show the presence of O–H or N–H band (3300–3500 cm–1) and a strong peak of C–O (1050–1150 cm–1), which confirms the degradation of the investigated dyes. In addition, the change of C=O (1715 cm–1) peak shape for RhB approves its degradation.[54] Notably, the ESI-MS spectra of MB after photocatalysis using bulk α-MoO3 or MoO3– (Figure S5) show almost the absence of the dye molecular weight at 284 m/z and formation of other species with high and low molecular weights.[72] Interestingly, in the case of photocatalysis using MoO3– nanosheets, several peaks of low molecular weights were observed, which confirm the dye degradation (Figure S5d). Some of these peaks are shown in Scheme S1.

Figure 6

Probing degradation product by FTIR: (a) FTIR spectra of RhB and (b) MB before and after photocatalysis. The highlighted peaks show the change of the dye function groups.

A comparison between photocatalysis using MoO3– nanosheets and other materials for dye degradation is tabulated in Table . Under UV irradiation, h-MoO3[36] and ZIF-8[54] show degradation efficiencies of about 71.0 and 82.3%, respectively, for MB. However, MoO3– nanosheets under visible light irradiation result in the disappearance of the characteristic absorption peak(s) of the investigated dye after 10 min. One can see from Table that the photocatalysis using MoO3– nanosheets has several advantages, such as (1) the use of a low-energy source (150 W), (2) the reaction occurs at room temperature, (3) the use of a low amount of catalyst, and (4) offering photocatalysis for a concentrated dye solution without losing the free radicals generated by MoO3–.[65] Furthermore, there is no dramatic change in the material crystallinity after catalysis (Figure S6). Therefore, MoO3– nanosheets are considered as an efficient photocatalyst and can be used for further applications including water treatment using membrane-based technologies.

Table 1

Comparison of Photocatalysis Using Different Materials

catalyst	catalyst and dye amount	photocatalysis conditions	degradation percentage	time (min)	refs
h-MoO3	100 mg of h-MoO3 was suspended in a 100 mL aqueous solution containing 10 mg/L of MB	visible light (350 W) and UV light (6 W)	67% under dark, 71% (UV) and 98% using visible	180	(36)
	25 mg of h-MoO3 was suspended in a 1 L aqueous solution containing 10 mg of MB	visible light irradiation with an E = 350 W and λ = 420 nm at ambient temperature	19%	105	(32)
	100 mg of MoO3 was suspended in 200 mL aqueous solution of RhB (5 mg/L)	CWAOa	81.7–97.6% (temperature, 10–100 °C)	10–30	(65)
ZIF-8	25 mg of ZIF-8 was dispersed into 50 mL of MB aqueous solution (10 mg/L)	500 W Hg lamp, and RTa	82.3%	120	(54)
MoO3–x nanosheets	8 mg of MoO3–x was dispersed into 20 mL of RhB or MB dyes (47.6 mg/L)	visible light (150 W), and RT	>45–80% with a complete absence of the characteristic peak of the dye	10	here

Catalytic wet air oxidation, CWAO; room temperature, RT.

Conclusions

In summary, we introduced a novel environmentally friendly exfoliation strategy to fabricate MoO3– nanosheets from commercially available bulk α-MoO3. FE-SEM, TEM, and AFM indicated that MoO3– possesses a nanosheet morphology with a thickness of about 10 nm. XRD, XPS, FTIR, and Raman spectroscopy confirmed the formation of MoO3– nanosheets. Visible light irradiation for the aqueous suspension of MoO3– nanosheets mediated LSPR peaks at 954 and 1160 nm, suggesting the creation of more oxygen vacancies upon light excitation, which increases the free carrier density. The MoO3– nanosheets displayed a satisfactory performance in dye decolorization under visible light irradiation for a short time (10 min). FTIR and ESI-MS confirmed the degradation of RhB and MB dyes after visible light photocatalysis in the presence of MoO3– nanosheets. The nanosheet morphology of MoO3– offers the potential for membrane technology and opens a new venue for the removal of dyes from waste water.

Experimental Section

Material and Methods

Bulk α-MoO3 was purchased from Alfa Aesar (Germany). RhB and MB were purchased from Sigma (Germany) and Fluka (India), respectively. All chemicals were used as purchased without any purification.

Synthesis of MoO3– Nanosheets

In a typical synthesis, a given amount (1.80, 0.90, or 0.45 g) of bulk α-MoO3 was refluxed in 200 mL of deionized water (DIW) for 5–7 days at 25–80 °C. The resulting suspension was then centrifuged at a speed of 10 000 rpm for 5 min. The unexfoliated MoO3 crystals were decanted from the dispersion of exfoliated MoO3– nanosheets. The faint yellow supernatant was collected and dried at 80 °C for 5–8 h to form green strips of MoO3– nanosheets.

Characterization Techniques

PXRD was performed in the 2θ range of 4°–60° using a PANalytical X’Pert Pro diffractometer equipped with a pixel detector and Cu Kα1 radiation (wavelength, 1.5406 Å). FTIR spectra were recorded by attenuated total reflectance FTIR (ATR-FTIR, UK, Varian 610-IR) in the wavenumber range of 4000–400 cm–1. Raman spectra were recorded using a LabRAM HR 800 Raman instrument with an 800 mm focal length spectrograph and wavelength of 786 nm. The XPS measurement was performed using AXIS Ultra (Kratos Analytical Ltd.) with monochromatic Al Kα radiation (hν = 1486.7 eV), and the binding energies were normalized to the C 1s peaks of (C–C) and (C–H) bonds (284.8 eV). The material morphology was observed using FE-SEM (JEOL, JSM-7401) and TEM (JEOL, JEM 2100, accelerating voltage 200 kV). The thickness of the nanosheets was determined by AFM (Bruker Nanoscope III in the tapping mode). The UV–vis–NIR measurements were acquired using a PerkinElmer Lambda 19 UV–vis–NIR spectrometer. ESI-MS was performed using Bruker solariX XR FTMS in the positive mode. Nitrogen adsorption isotherms were obtained at 77 K using a Micromeritics ASAP 2020 instrument. Before the measurements, the samples were degassed under a reduced pressure (<10 Pa) at 100 °C for 3 h.

Visible Light Activation of MoO3– Nanosheets

The aqueous suspension of MoO3– nanosheets (10 mL, 8 g/L) was irradiated with visible light (Halolux Ceram Eco 150W) under ambient temperature. The UV–vis–NIR absorption of the suspension was recorded in the wavelength range of 200–1300 nm.

Photocatalytic Decolorization of Organic Dyes

The photocatalytic activity of MoO3– nanosheets was evaluated using RhB and MB as models. In a typical experiment, 1 mL of the dye stock solution (1 g/L) was diluted to 20 mL, and then 1 mL of the catalyst suspension (bulk α-MoO3 or MoO3– nanosheets) with a concentration of 8 g/L was added. The reaction mixture was stirred under ambient temperature and then irradiated with visible light (Halolux Ceram Eco 150W, the distance between the reaction vials and the light source was ∼25 cm). After a given reaction time, a sample of 0.1 mL from the reaction mixture was diluted to 4 mL by DIW, and then its UV–vis absorption spectrum was recorded in the range between 300 and 800 nm. The removal efficiency of the dye was estimated according to the following equation (eq )where A0 is the initial absorbance of the dye before mixing with the catalyst and A is the absorbance of the dye at a given reaction time t (min).

The effect of direct addition of the catalyst on dye decolorization was measured by adding 50 μL of MoO3 or MoO3– (8 g/L) to 4 mL of the solution containing 100 μL of the dye (1 g/L).